The need for a self-watering system for plants is well established, since agriculture utilizes approximately 70% of the world's fresh water resources, and many products have been designed and built to satisfy this need to varying degrees. Some systems supply a small continuous amount of water, often referred to as drip irrigation or trickle irrigation, which supply water to the root zone irrespective of the plants' needs. Other systems rely on the moisture level in the soil to signal the need for water. Still others use wicks that bring water to the plant as a result of surface tension and the capillary rise effect.
Drip irrigation or trickle irrigation is a well-established method of growing crops in arid areas. It is claimed to be 90% efficient in water usage compared to 75-85% for sprinkler systems. The basic drip irrigation system generally consists of a surface tube from which small dripper tubes/emitters are fitted to take water from the supply tube to the roots of the plant on either side of the supply tube. The dripper tube/emitter limits the flow of water to the roots drop by drop based on the viscous resistance to water flow within the emitter/dripper tube. The drip rate is determined by the calculated needs of the specific plants, the soil conditions, anticipated rain fall, and evapotranspiration rate, and can vary from 1 to 4 L/hr per plant.
The need to estimate the water requirements of the crops or the amount of nutrients to be supplied in the water is seldom exact and invariably leads to wastage of water. It was shown that the roots of plants can control the release of water that is stored behind a thin porous hydrophilic membrane that is believed to become hydrophobic due to the adsorption of organic impurities in the water. The mechanism is not fully understood, though it has been speculated that among the root exudates is a surfactant that opens the pores of the membrane that became hydrophobic due the adsorbed organic impurities in water. The hydrophobic membrane inhibits the flow of water to the plants. However, the roots of the plants exude a variety of chemicals that include a surfactant that open the pores of the membrane by making it hydrophilic. Thus water can now flow to the roots and the membrane becomes hydrophobic when the plant has had enough water.
It has also been shown that when two reservoirs (one with water and the other containing nutrient solution) with membranes are presented to a plant, the plant can distinguish between the two sources, taking as much water as it needs and as much nutrients as it requires. The ratio of water to nutrient can vary from 2-5 to 1 depending on the concentration of the nutrient solution.
Several sub-surface systems have been developed that include tubes that are porous or are perforated to permit the continuous slow release of water. However, these hydrophobic tubes, which require a water pressure of up to two atmospheres, do not automatically stop the delivery of water when the plants have had enough or, for example, when it rains.
One possible reason for the absence of a commercial irrigation system using the membrane system may be the difficulty of obtaining a membrane that can supply the necessary amount of water for new plants or seedlings as well as a fully grown and mature plant that is sprouting and producing fruit and produce. Another possible reason may be the reliance on constant trace amounts of organic solutes in the water, which become adsorbed on the exit walls of the hydrophilic pore channels of the membrane, converting the membrane into a hydrophobic system, which then stops or greatly reduces the flow of water through the membrane. Another reason may be the difficulty of obtaining hydrophilic tubes of suitable wall thickness and diameter that are sufficiently durable to make the process economical.
The Russian SVET space plant growth system consists of a box greenhouse with 1000 cm2 growing area with room for plants up to 40 cm tall. The roots were grown on a natural porous zeolite, with highly purified water keeping the roots at the required moisture level. Zero-gravity growth chambers used by NASA have included a microporous ceramic or stainless steel tube through which water with nutrient is supplied to irrigate the greenhouse plants. Systems using porous ceramic, stainless, or hydrophobic membranes to deliver water and/or nutrients to plants are basically a form of drip irrigation where the water/nutrients are always delivered whether the plants need it or not. As will be apparent to one of skill in the art, the ceramic or stainless tubes are thicker and the organic components are adsorbed onto the full length of the channels and cannot be removed by the plant's exudates.
FIG. 7 shows the flow of water and nutrient solution for a single plant. FIG. 7, in particular, is a daily record of water flow (in mL/day) through 12 cm2 of microporous Amerace A-10 fitted to the bottom of two 285-mL identically sized and shaped reservoirs (No. 1 for water and No. 2 for nutrient solution) that were embedded in the potting soil of a well-established Ficus indica (insert), showing the effect on the pattern of water flow when (i) root contact with the membrane was established, and (ii) when the total flow ceased to be greater than the rate of water uptake (after day 24). In general, the flow of water is about three times larger than from nutrient solution. It has been shown that a change in the concentration of the nutrient alters the ratio of flow from the two reservoirs. In FIG. 7, the exudates from the plant's roots convert step 3 back to step 1 in FIG. 8. This has been shown in an experiment by allowing a membrane to close after a specified volume of water was passed through an Amerace-10 membrane. The exit side of the membrane was then washed with alcohol and the water flow through the membrane resumed and eventually stopped when all the alcohol was washed away and the organic impurities were allowed to be adsorbed onto the exit wall of the pores shown in FIG. 8.
Again referring to FIG. 8, in step 1, as water leaves the pore of the membrane, it spreads out onto the membrane's surface, which is hydrophilic. A large drop forms and leaves the surface. As the surface becomes coated by the adsorbed hydrophobic impurities in water, the water leaving the capillary pore of the membrane cannot spread out over the surface and a smaller drop can be formed (step 2). When further coating continues, there is no room for the water to spread out onto the surface and a greater force is required to push the water through the hydrophobic area shown in step 3. The membrane is converted from the hydrophilic state to a hydrophobic state. It is made hydrophobic by the adsorption of the organic impurities in the water and/or nutrient solution. This closes the pores and prevents water from leaving the membrane under the prevailing pressure conditions. If the pressure is increased, it becomes possible for the liquid to flow again because the surface tension of water no longer can prevent the water from breaking through the pores.